Without limiting the scope of the invention, its background is described in connection with previously used techniques of time etching silicon dioxide and polysilicon.
Controlling the thickness of thin films is of prime importance in the fabrication of semiconductor devices. As the minimum feature size of devices such as transistors decreases, the tolerances on the film properties also must decrease. At present, the tightest tolerances encountered in advanced CMOS devices are related to the thickness of the oxide gate layer, and are typically held to about .+-. three (3) .ANG. (angstroms). Heretofore, ex situ thin film analysis has been routinely used to monitor film thickness and to provide the feedback concerning a fabrication process. This is true, since automatic and fast in situ analysis of the film and the fabrication process has not been available. Although a fast in situ analysis would be much more desirable for accurate and precise control, there has been no known successful use of ellipsometry for measurement of the in situ film thickness of semiconductor devices in real-time. There are several reasons which have discouraged such in situ real-time use, including the difficulty of determining the desired thickness parameter from the Psi (.PSI.) and Delta (.DELTA.) parameters, which are more readily available through ellipsometric techniques. Further, the speed at which Psi (.PSI. ) and Delta (.DELTA.) parameters' measurements can be obtained was entirely too slow.
Ellipsometry is an optical technique which measures the change in polarization of light upon reflection or transmission of the light through a film or interface. This change is characterized by the change in amplitude, Psi (.PSI.), and the change in phase, Delta (.DELTA.) of the light polarized in the plane parallel to the plane of incidence relative to the light polarized in the plane perpendicular to plane of incidence. The ellipsometric parameters Psi and Delta are sensitive to the thickness and refractive indexes of the films and the substrate as well as the wavelength and the angle of incidence of the light beam. Consequently, ellipsometry does offer the prospect of measuring in situ film thicknesses to the accuracy and precision necessary without perturbing the process system or physically contacting the wafer. To date, using the process of this invention, films have been produced which have reproducibility in their thicknesses between .+-. three (3) .ANG.. Further, it is anticipated that film thickness accuracy and precision of up to .+-. one (1) .ANG. may be possible using the techniques of this invention.
Therefore, it is an object of this invention to provide methods and apparatus for achieving in situ measurement techniques which can produce thin films having a deviation of no more than .+-. three (3) .ANG..